Stoker control system



Nov. 4, 1941. A. H. LO CKRAE STOKER CONTROL SYSTEM Filed DeOQ 13, 1937 2 Sheet's-Shet 1 IINVENTOR Arlhur ILLockrae BY A/ 2 ATTORNEY Nov. 4, 1941.

A. H. LQCKRAE STOKER CONTROL SYS TEM Filed Dec. 15, 1937 2 Sheets-Sheet 2 IOI INVENTOR ur HJJocKrue ATTORNEY Patented Nov. 4, 1941 STOKER CONTROL SYSTEM Arthur H. Lockrae, Minneapolis, Minn., assignor to Minneapolis-Honeywell Regulator Company, Minneapolis, Minn., a corporation of Delaware Application December 13, 1937, Serial No. 179,501

'11 Claims.

This invention relates to a stoker control system, and more particularly to one wherein thermo-couples located within the furnace are in control of the stoker motor.

It is customary to control the operation of a stoker motor by a temperature responsive device located in a space to be heated, as well as by means responsive to conditions within the furnace. Were a stoker controlled only by a room thermostat, there would be times during the heating season in mild weather when the room thermostat would call for heat, and consequently cause operation of the stoker, only at infrequent intervals. If the stoker operated only in response to a demand for heat within the space, the combustion in the furnace might reach such a low point between operations of the stoker that the fire might burn out. customary to provide a device responsive to temperature conditions within the furnace, as for example, in the stack to cause operation of the stoker whenever the temperature in the furnace reaches such a low point that a need for the addition of more fuel to maintain the fire, is indicated. It is also customary to provide a second temperature responsive device which prevents operation of the stoker whenever the temperature within the furnace reaches such a low point that it indicates that the fire has burned out. This prevents operation of the stoker upon a call for heat in the space, when the fire has burned out for some reason or other, since obviously the stoker should not be operated when the fuel being supplied to the furnace will not be ignited.

A more accurate way to control the operation of the stoker is to provide a device responsive to conditions in thefire-bed instead of a device responsive to conditions elsewhere in the furnace and my invention is concerned with the use of such means. By the use of thermo-couples located in proximity to the fire-bed, one of which controls the stoker when fuel is necessary to support combustion and another which prevents operation of the stoker when the fire is so low that further combustion cannot take place even if additional fuel is supplied, the stoker may be very accurately controlled. These thermo-couples may energize relays which directly control the operation of the stoker motor and while these thermo-couples must supply sufllcient energy to hold the relay armatures in, they need not supply enough energy to pull the armature in. I have therefore provided unique means to oper- For this reason it is the stoker when the addition of fuel is necessary to support combustion and to cause operation of the stoker at these times for predetermined periods only, in a manner to be hereinafter set forth.

It is also possible to provide a single thermocouple in combination with a thermal timer to control the stoker. This thermo-couple may be of a type which creates suflicient energy to pull the relay armature in as well as to hold the armature in. When combustion falls to such a point as to indicate a need for additional fuel, the relay armature drops out and causes operation of the stoker. If combustion does not then increase to such a point that the relay armature is pulled in under the influence of the thermocouple within a predetermined period of time, it is an indication that combustion has dropped to such a point that the addition of further fuel will maintain combustion. The thermal timer maytherefore be energized when the relay controlled by the therrno-couple is deenergized, and if the relay remains deenergized for a predetermined period of time, the thermal timer opens the circuit to the stoker, thereby preventing further operation thereof, the switch operated by the timer being of the manual reset type.

It is therefore an object of my invention to provide an improved stoker control system.

A further object of my invention is to provide an improved stoker control system wherein the operation of the stoker motor is controlled by one or more thermo-couples suitably located prevents operation of the stoker whenever the temperature of the furnace drops to a value so low that it indicates that the fire has substantially burned out and can no longer support combustion.

Another object is tolprovide a stoker control system wherein a thermo-couple is suitably located within a furnace for causing operation of the stoker whenever the temperature in the furnace'jdrops to a point indicative of a condition wherein additional fuel must be supplied to support combustion, and wherein the stoker conate the relay which controls the operation of tinues to operate until combustion increases to a desired value, but should this value not be attained within a predetermined time, a manual reset switch opens the circuit to the stoker under the influence of a thermal timer, this time being so chosen that if combustion has not risen to the desired value within this period of time, it is an indication that the fire has dropped so low that it cannot support further combustion, and further operation of the stoker is impossible until the above mentioned switch has been manually reset.

A further object is to provide a novel control means for a relay, wherein a thermo-couple is provided to cause the energization of said relay, and other means are provided to move the relay and to hold the relay in energized position for a certain period, at the end of which period the relay will remain in its energized position if the temperature at which the hot junction of the thermo-couple is located has reached a sufficiently high value.

Other objects will become apparent upon a study of the specification and claims together with the appended drawings in which are illustrated two embodiments of my system and in which:

Figure 1 illustrates in diagrammatic form one embodiment of my invention, and

Figure 2 illustrates a second embodiment of my invention.

Referring more particularly to Figure 1, a section of a furnace indicated generally by the reference character H3 is illustrated. This furnace comprises an outer jacket II and an inner jacket l2, a grate |3 for supporting the fuel to be burned, and a delivery pipe H for feeding-fuel to the furnace and onto the grate. Within the pipe I4 is a feed screw |5 for feeding the fuel through the pipe and to the furnace, this feed screw being driven by means of a motor IS. A hopper I1 is provided for holding the fuel to be fed to the furnace.

Within the furnace and preferably located in proximity to the fire-bed are a pair of thermocouples 20 and 2|. While it is preferable that these thermo-couples be located near the firebed it will be understood that they may be located in any suitable part of the furnace wherein a change in temperature indicates a change in the condition of the fire. These thermocouples each comprise two different kinds of metals having different thermal characteristics and joined together at one end, these junctions being termed the hot junctions and being located within the furnace.

Thermo-couple 20 controls the energization of a relay generally indicated by the reference character 25, this relay including a winding 26 and an armature 21 pivoted at l8. A second relay 28 including a coil 29 and an armature 30 pivoted at I9 is controlled by the thermo-couple 2|. As illustrated, thermo-couple 2| is located near the inside wall of the furnace or away from the hottest part of the fire so that when the temperature at this point drops to a certain predetermined value, the thermo-couple will cool down and cause operation of the stoker, as will be hereinafter described, to supply enough fuel to support proper combustion. Thermo-couple is located near the entrance of the feed pipe sired, be located at the same point within the furnace and be so constructed that they will respond to different temperatures at said point to properly control the stoker motor I6. These thermo-couples do not create sufllcient energy to pull in armatures 21 and when the hot junctions are heated up, but they do furnish sufllcient energy when heated up to maintain armatures 21 and 30 pulled in if they have been moved to this position by some other means. Since relay 25, as will be hereinafter explained, must be pulled in, in order to permit operation of the stoker, it will be apparent that upon starting the fire this relay must be held in by some external means until the fire has reached such a point that the thermo-couple 20 will hold it in. This means for holding the relay in at this time forms no part of my invention and is therefore not being illustrated.

Located in a space being heated is a thermostat generally indicated by the reference character 32, this thermostat including a bimetallic element 33, a flexible blade 34 cooperating with a fixed contact 35 and a relatively stiff blade 35 cooperating with a fixed contact 31. Arm 34 is spaced closer to contact 35 than is arm 36 with relation to contact 31. Upon a drop in temperature arm 34 first engages contact 35 and upon a further drop in temperature this arm will fiex and permit the engagement of arm 35 with contact 31. Upon a rise in temperature in the space, the arms will move out of engagement with their respective contacts in the reverse order.

Under the control of the thermostat 32 and also the relay 28 is a relay indicated generally by the reference character 40. This relay includes a coil 4|, an armature 42, arms 43, 44, and cooperating with contacts 46, 41, and 48, respectively. When said relay is deenergized the arms are in the positions illustrated, and upon energization of the relay, the arms move into engagement with their respective contacts.

Arm 43 is made long enough to engage the armature 30 of relay 28, if this relay is in its deenergized position as illustrated. It will be noted that armature 30 includes an inclined surface 55 and when arm 43 moves into engagement with surface 55 it causes movement of the armature 30 into engagement with the core of the relay 28. Contact 51 is operated by the armature 30, and upon upward movement of said armature the contact 51 is moved out of engagement with the fixed contact 53.

A bimetallic element is provided for additionally controlling relay 40, this elefnent including contacts 5| and 52 which cooperate with fixed contacts 53 and 54, respectively. In series withthe contact 54 is a heater element 55, the function of which will be later described.

Suitably mounted in the bonnet or other suitable portion of the furnace is a bimetallic element 50 which operates a mercury switch 8| by means of a rod 62 in a manner well known in the art. Switch 5| includes a mercury element 53 and contacts 64 and 55. This switch is normally in the position illustrated wherein the contacts 84 and are connected by the mercury element 53, and upon attainment of excessive temperature in the bonnet of the furnace, the bimetallic element causes the switch 5| to be tilted in the other direction thus breaking the circuit through the contacts 54 and 55.

Line wires 10 and 1| are provided for enersizing the motor it, these lines being connected to a suitable source of power (not illustrated). Connected to the lines I8 and 'II by means of conductors I2 and I3, respectively, is a line voltage primary 14 of a step-down transformer I5. This transformer also includes a low voltage secondary I6 which functions to energize the various relays.

Operation of Figure 1 Assume that the fuel within the furnace I8 is-in a state of combustion and is sufliciently hot so that the relay 25 is energized but is not hot enough to cause energization of relay 29 by the thermo-couple 2|. This conditionindicates that more fuel should be supplied to the furnace. Since relay 28 is in .its deenergized position, contacts 51 and 58 are in engagement with one another and thereby cause the relay 4| to be energized through the following circult: from one side of the secondary I6 of transformer I5 through conductors 88, 8|, contacts 82 and 83, conductors 84 and 85 through the heating element 55, conductor 86, contacts 54, 52, the bimetallic element 58, contacts 5| and 53, conductor 81, contacts 58, 51, conductors 88, 89, 88, coil 4| of relay 48, and conductor 8| to the other side of secondary I6. Energization of coil 4| causes arms 43, 44, and 45 to move into engagement with contacts 46, 41, and 48, respectively.

Movement of arm 43 into engagement with contact 46 causes armature 38 of relay 28 to move upwardly, thus opening contacts 51 and 58 and breaking the aforedescribed energizing circuit for the coil 4|. Movement of arm 44 into engagement with contact 41 establishes a holding circuit for the relay 4 I this circuit being through the heater 55 and bimetallic element 58 as follows: from the secondary 16 through conductors 88, 8|, contacts 82 and 83, conductors 84, 85, heater 55, conductor 86, contacts 54, 52, b metallic element 58, conductor 85,. contact 41, arm 44, conductors 83, 88, relay coil 4| and conductor 8| to the other side of the secondary 16. It will therefore be apparent that as long as this holding circuit is established the relay 48 will remain energized. After a predetermined time interval however the heating element 55 will attain such a temperature that it will cause bimetallic element 58 to be flexed, thereby moving contact 52 out of engagement with contact 54 and breaking this holding circuit.

Movement of arm 45 of relay 48, upon energization of the same, into engagement with contact 48 causes a circuit to be established through the stoker motor I6 as follows: from the line II through conductor 86, arm 45, contact 48, conductor 91, contacts 64 and 65 of switch 6|, conductor 88, terminal 89 of themotor I6 through the motor to terminal I88 and conductor IM to the line 18.. Fuel will now be fed to the furnace I8 through the pipe I4 as long as relay 48 remains energized. At the end of a certain peri-' 0d, the heater 55 will cause relay 48 to be deenergized as explained above, thus causing the arms to move back to their original positions and deenergizing the stoker motor I6. temperature of the furnace has reached such a point that thermo-couple 2| causes energization of the relay coil 29, armature 38 will remain in its raised position, thus maintaining contacts 51 and 58 in their open position. If the temperature of the furnace has not reached a sufficient value, armature 38 will return to the position illustrated, contacts 51 and 58 will reengage, and

the contacts of bimetallic element 58 will engage their respective contacts 53 and 54 and the above described cycle will be repeated.

Assume now that the relay armature 38 is held in, but the temperature of the space being heated falls to a value which causes arms 34 and 36 to engage contacts 35 and 31, respectively. A circuit is now established through the relay coil 4| as follows: from the secondary I6 through conductors 88, 8|, contacts 82, 83, conductor 84, resistance element I85, conductor I86, contact 35, blades 34 and 36 of thermostat 32, contact 31, conductors I81, 88, 89, 88, relay coil 4| and conductor 9| to the other side of the secondary I6. A circuit is now established through the relay 48 which is independent of the engagement of arm 36 with contact 31 so that said relay will If the i after the heating element has cooled sufliciently not be deenergized until arm 34 has moved out of engagement with contact 35, thus providing proper operating differential for the system. This holding circuit is as follows: from the secondary I6 through conductors 88, 8|. contacts 82 and 83, conductor 84, resistance element I85, conductor I86, contact 35, arm 34, bimetallic element 33, conductor 8, contact 46, arm 43, conductor 98, coil 4| and conductor 8| to the other side of secondary I6. It will be noted that as long as or as soon as the relay is energized a second holding circuit through the heating element 55 and bimetallic element 58, similar to that described above, is established through the relay coil 4|. It would be undesirable to have this holding circuit remain established after the room thermostat is completely satisfied and for this reason the resistance I85 is providedin the circuit through the thermostat 32 so that sufllcient current will pass through heater 55 during the existence of these parallel paths through the relay, so that the circuit through the heater 55 will be broken before the room thermostat is satisfied, thus assuring that the relay 48 will be deenergized as soon as the room thermostat 32 is satisfied.

Since the circuit through the stoker motor I6 is through the switch 6|, it will be apparent that should the temperature in the bonnet of the furnace reach an excessive value before the room thermostat 32 is satisfied, this switch will cause the motor l6 to be deenergized and thus interrupt the supply of fuel to the furnace until the temperature therein drops to a safe value.

If for any reason the fire in the furnace I8 should get so low that combustion cannot besustained' any longer, even though more fuel be added, the hot joint of thermo-couple 28 will cool down sufflciently so that relay 25 becomes deenergized and armature 21 drops out, thus causing contact 82 which is actuated thereby to move out of engagement with contact 83. Since all the circuits through the relay 48 must pass through these contacts, the opening of the same will obviously cause therelay 48 to become deenergized, thus preventing operation of the stoker motor I6. In order to again put the system in operation a new fire must be started and relay 25 will be manually held in until the temperature in the furnace reaches a value high enough so that thermo-couple 28 will hold relay 25 in.

It will therefore be seen that with this system, operation of the stoker by the thermostat is supplemented by thermo-couples 28 and 2| controlling relays 25 and 28, respectively, so that the stoker will be operated whenever addition of more fuel to the furnace is necessary to support combustion and to prevent operation of the stoker motor whenever the fire in the furnace goes out. I am thus able to obtain the same results which have heretofore been automatically attained only by the use of more complicated and elaborate equipment.

Description of Figure 2 Referring now to Figure 2, a single thermocouple I20 is provided in the furnace I0. This thermo-couple may be located at any suitable place in the furnace, as for example, near the entrance of feed pipe I4, or at a location near the fire-bed which is ordinarily hot if proper combustion is being supported, but should the fire-bed become cool at this point it indicates a need for the addition of further fuel. This thermo-couple controls the energization of a relay I2I, this relay comprising a winding I22, a core I23, and an armature I24. connected to the thermo-couple I20 by means of conductors I21 and I28. gages a contact I25 when the relay I2I is energized, but upon deenergization of the relay, the armature I24 drops into engagement with a contact I26. Thermo-couple I20 is of such a character that when the junction thereof is sufficiently hot, sufficient energy is created thereby to move the armature I2I into engagement with the core I23 and hold it in this position as long as the hot junction of the thermo-couple is at a sufficiently high temperature.

A manual reset thermaltimer I30 is arranged to be heated when the relay I2I is deenergized as will be hereinafter set forth. This thermal timer includes a heater I3I, a bimetallic element I32, a switch arm I33, and contacts I34 and I35 which are bridged by the arm I33 when in the position illustrated. Spring I36 biases the arm I33 in a direction to interrupt the flow of current through contacts I34 and I35. It will be apparent that as the heater I3I heatsup, bimetallic element I32 is gradually moved upwardly, and after a predetermined time moves out of engagement with the arm I33, which arm is then moved by the spring I36 out of engagement with contacts I34 and I35. A handle I31 is provided for moving the arm I33 into engagement with contacts I34 and I35.

' The operation of the stoker motor I6 is normally controlled by a room thermostat 32 which may be of the same type as that illustrated in Figure 1, said thermostat including bimetallic element 33, contact arms 34 and 36 cooperating with contacts 35 and 31, respectively, in the manner set forth in the description of Figure l.

A relay 40, the energization of which causes operation of the stoker, is provided, this relay comprising a coil M, an armature 42, and arms 43 and 45 cooperating with contacts 46 and 46 as in Figure 1.

A limit switch 6I similar to that shown in Figure 1 is provided for interrupting the circuit to the stoker motor upon the attainment of an excessive temperature within the furnace.

Operation Figure 2 The operation of the system disclosed in Figure 2 is as follows: assuming the temperature within the fire-bed at the location of the thermo- Coil I22 is Armature I 24 en- Should the temperature within thespace being heated fall to such a value that contacts 35 and 31 are engaged by arms 34 and 36, relay 40 will be energized by the following circuit: from one side of the secondary 16 of transformer through conductor I40, contact I35, arm I33 of the thermal timer I30, conductor HI, 9, resistance I42, conductor I43, contact 35, arms 34 and 36 of the thermostat 32, contact 31, conductor I44, contact I of relay I2I, arm I24, conductors I45, I46 through relay coil H and conductor I41 to the other side of secondary 16. Arms 43 and 45 of relay 40 are now caused to move into en gagement with contacts 46 and 48, respectively. Engagement of arm 43 with contact 46 creates a holding circuit for relay coil 4| which circuit is as follows: from one side of the secondary 16 of transformer 15 through conductor I40, contact I35, arm I33, conductor I4I, resistance I42, conductor I43, contact 35, arm 34 of thermostat 32, bimetallic element 33, conductor I50, contact 46, arm 43, conductors I5I, I46, coil H of relay 40 and conductor I41 to the other side of secondary 16. Relay 40 accordingly remains energized until arm 34 of thermostat 32 moves away from contact since this holding circuit is independent of the engagement of arm 36 and contact 31.

Current now flows through the stoker motor I6 by the following circuit: from the line H, conductor 96, arm 45, contact 48, conductor 91, contact 64, mercury element 63, contact 65, conductor 98 terminal 99 of motor I6 through the motor to terminal I00 and conductor IOI to the line 10. The stoker now starts operating and feeding coal to the furnace I0 until the temperature of the space being heated reaches a predetermined value whereupon the arm 34 moves away from contact 35 thus interrupting the operation of the stoker.

Should the fire get so low during a time that the room thermostat is not calling for heat that the supply of additional heat is necessary to support combustion, then relay I2I becomes deenergized by reason of the fact that the junction of thermo-couple I20 cools to a point where it does not furnish sufficient energy to the relay to hold it in energized position. Relay coil 3| will now be energized as follows, it being understood that arm I24 of the relay I2I is in engagement with contact I26: from secondary 16 of transformer 15 through conductor I40, contact I35, arm I33, contact I34, conductor I60, heater I3I, conductor I BI, contact I 26, arm I 24, conductors I45, I46, relay coil H, and conductor I41 to the other side of secondary 16, Energization of relay causes operation of stoker motor l6 as previously described. The flow of current through the heater I 3I will cause the temperature of said heater to start rising and if the flow of current through this heater continues for a predetermined period, as for example twenty minutes, the bimetallic element I32 will be moved upwardly to release the arm I33 and interrupt the aforementioned energizing circuit for the relay 40, This condition will exist only when the fire has dropped to such a low point that combustion will not be sustained even with the addition of further fuel. The period of time it takes heater I3I to cause the release of arm I33 is so chosen that if there is proper combustion within the furnace, relay I 2| will be energized by the thermo-couple I20 before said arm is released. If the addition of fuel to the furnace raises the temperature to a sufficient value,

thermo-couple I20 will cause the energization of relay I2I whereupon arm I24 moves out of engagement with contact I26 and back into engagement with contact.l25, thus interrupting the circuit through the heater I3I and placing the stoker again under the control of thermostat 32. If, however, the fire does not come up sufliciently, the addition of further fuel will be ineffective to maintain the fire, and the release of arm I33 from engagement with contacts I34 and I35 by the heater I 3I will interrupt the circuit to the relay 40 thus causing stoker motor I6 to stop. Since the circuit to relay 40 which is energized by the thermostat 32 is dependent upon the engagement of arm I33 with contact I35, it will be clear that when this arm moves away from contact I35, the operation of stoker motor I6 will be discontinued even though the thermostat 32 should later start calling for heat in the space.

' In order to start the system up again it will be necessary to manually move arm I33 back into engagement with contacts I34 and I35 which will be done by the attendant after a new fire has been built within the furnace I0,

If at the time that relay I2I is deenergized indicating the necessity for adding fuel to the fire, the room thermostat should also call for heat it will be noted that besides the energizing circuit for relay 40 which includes the heater I3I there is also a circuit through the previously described holding circuit through coil II which includes the thermostat 32. Resistance I42 is accordingly provided in this circuit so that the heater I3I may draw sufiicient current to properly heat up within the predetermined time, this resistance corresponding to resistance I05 described inconnection with Figure 1.

It will accordingly be seen that in this form of the invention, results similar to those obtained in the form shown in Figure 1 are produced with the use of a single thermo-couple and a single thermo-couple relay. The operation of the stoker is normally under the control of the thermostat 32, the operation of the stoker when additional fuel is required in the furnace during periods when the room thermostat is not calling for heat is controlled by the thermo-couple I20, and the shut-down of the system during such times that the fire is out or is too low to support combustion is controlled by the thermal timer I30. It will accordingly be seen that I have provided a very simple and effective system for controlling a stoker by the use of a minimum of parts and I one which is relatively inexpensive to install.

Having described the preferred forms of my invention it will be apparent to those skilled in the art that it is capable of further modifications and I wish it to be understood that my invention is limited only by the scope of the appended claims.

I claim as my invention:

1. A control system for a motor operated stoker comprising a relay, a second relay, means responsive to energization of said second relay for energizing said motor, means responsive to deenergization of said first relay for causing the energization of said second relay, means respon sive to'energization of said second relay for causing said first relay to move to energized position and to stay in said position as long as said sec- 0nd relay is energized, a holding circuit for said second relay to hold the same energized for a predetermined period only after it has been energized in response to the deenergization of said first relay, and means responsive to the attainment of a predetermined condition for energizing said first relay.

2. In a system of the class described, a fuel burning apparatus, means for feeding fuel to said apparatus, control means for said feeding means including a first relay, a second relay, means responsive to energization of said second relay for causing operation of said feeding means, means responsive to deenergization of said first relay for causing energization of said second relay, means responsive to energization of said second relay for causing said first relay to be moved to energized position and to be maintained in said position as long as said second relay is energized, means creating a holding circuit for said second relay for a predetermined time only upon energization of the same, means responsive to the attainment of a high predetermined temperature in said apparatus for causing energization of said first relay, and additional means responsive to the attainment of a low predetermined temperature in a space to be heated for causing energization of said second relay independently of the first relay.

3. In a system of the class described, a thermocouple including a hot junction subjected to varying degrees of temperature, a relay, means causing said relay to be energized by said hot junction upon the attainment of a predetermined high temperature to which said junction is subjected, said thermo-couple creating insufficient energy to pull the relay armature in but creating sufficient energy to hold the armature in after it has been pulled in, a second relay, means responsive to deenergization of said first relay to energize said second relay and means responsive to energization of said second relay to cause the first relay to be put in energized position.

4. In a system of the class described, a therm0 couple including a hot junction subjected to varying degrees of temperature, a relay, means causing said relay to be energized by said hot junction upon the attainment of a predetermined high temperature to which said junction is subjected, said thermo-couple creating insufficient energy to pull the relay armature in but creating sufficient energy to hold the armature in after it'has been pulled in, a second relay, means responsive to deenergization of said first relay to energize said second relay, means responsive to energization of said second relay to cause the first relay to be put in energized position, and means for holding the second relay in energized position for a predetermined time after the first relay has been put in its energized position.

5. In a system of the class described, a first relay, a second relay, condition responsive means for causing the energization of the second relay, means responsive to the deenergization of the first relay for causing the energization of the second relay, a second condition responsive means in control of the first relay, and means operative when the second relay is energized in-response to the deenergization of the first relay for causing the second relay to be deenergized after a predetermined time.

6. In a system of the class described, fuel burnapparatus, means for feeding fuel to said apparatus, a first relay, means responsive to energization of said relay for causing operation of said fuel feeding means, means responsive to the temperature of a space being heated for controlling the energization of said first relay, a second relay, means responsive to deenergization of said second relay for causing energization of said first relay independently of said temperature responsive means, a thermo-couple adjacent the fuel burning apparatus in control of said second relay and arranged to maintain the second relay energized unless the temperature to which said thermocouple is subjected drops to a predetermined value whereupon the second relay becomes deenergized, timing means in control of the first relay when said relay is energized by reason of the deenergization of the second relay, means responsive to deenergization of said first relay for causing the energization of said timing means, and means operated by the timing means for causing said first relay to remain energized in response to deenergization of the second relay for a predetermined time only.

7. In a system of the class described in combination, a fuel burning device, control apparatus comprising a thermostat and relay means controlled thereby for controlling the fuel burning device, electromagnetic means cooperating with said relay means in controlling said luel burning device, a thermo-couple responsive to heat of burning fuel controlling said electromagnetic means, said electromagnetic means being movable to energized position by said relay means, and timing means operable to begin a timing period upon said electromagnetic means being moved to energized position and said timing means governing said relay means in a manner such that after a predetermined period said electromagnetic means becomes dependent upon said thermocouple for remaining in energized position.

8. In a system of the class described in combination, a fuel burning device, a thermostat and a relay energizable thereby controlling said fuel burning device, an electromagnetic device including an armature and having contacts for control ling said relay, a thermo-couple responsive to heat of burning fuel for controlling said electromagnetic device, means whereby said relay is operable to move said armature into energized position of the electromagnetic device, timing means operable to start a timing period whenever said relay is operated in response to said electromagnetic device, and said electromagnetic device and timing means periodically operating said relay if the said armature is not retained in energized position of the electromagnetic device in response to the thermo-couple.

9. In a system of the class described, in combination, a fuel burning device, a thermostat and a relay energizable thereby controlling said fuel burning device, an electromagnetic device including an armature and having contacts causing energization of said relay when the electromagnetic device is energized, a thermo-couple responsive to heat of burning fuel for energizing said electromagnetic device, means whereby said relay is operable to move said armature into energized position of the electromagnetic device when the relay is energized, said relay becoming energized through said contacts when said electromagnetic means is deenergized, a timing device having contacts in the circuit energizing said relay when said electromagnetic device is deenergized, means forming a maintaining circuit for said relay independent of said first mentioned contacts, said timing device controlling said maintaining circuit and operable to interrupt it to deenergize said relay at a predetermined time after said relay has become energized in response to deenergization of said electromagnetic device, said contacts of said timing device remaining open for an interval of time during which said relay does not become reenergized even though said electromagnetic device is deenergized whereby said relay is periodically energized at spaced intervals during the time said electromagnetic device is deenergized.

10. In a solid fuel burning apparatus, means for feeding solid fuel, means responsive to the attainment of a predetermined low temperature in said apparatus for immediately starting said feeding means, and timing means for causing said feeding means .to operate for a given period of time independently of said responsive means after having been started, whereby the feeding means are not started unless said low temperature is attained and are then operated for a period of given duration.

11. In a solid fuel burning apparatus, means for feeding solid fuel, means responsive to the attainment of a predetermined low temperature in said apparatus for immediately starting said feeding means, and timing means for causing said feeding means to operate for at least a given period of time after having once been started, said timing means being arranged to repeatedly operate said feeding means for said given periods of time so long as said temperature remains at said predetermined low value.

ARTHUR H. LOCKRAE. 

